Recently, demand for a liquid crystal panel has gotten higher as not only direct-view type but also projection type display element such as projection TV. In the case where a liquid crystal display panel is used as the projection type display element, as an enlargement ratio is raised by using a constant number of picture elements, roughness of a picture becomes more noticeable. In order to obtain a precise image even at a high enlargement ratio, it is necessary to increase a number of picture elements.
However, if a number of picture elements of a liquid crystal panel is increased, especially in an active matrix type liquid crystal, an area of parts other than picture elements becomes relatively large, and an area of a black matrix which covers the above portion is increased. As a result, an area of the picture element which contributes to display is decreased, so an aperture ratio of the display elements drops. If a drop in the aperture ratio occurs, a picture becomes dark, thereby deteriorating quality of an image.
In order to prevent such a drop in an aperture ratio due to increase in a number of picture elements, Japanese Unexamined Patent Publications No. 60-165621, 165622, 165623, 165624, 262131/1985 (Tokukaisho 60-165621, 165622, 165623, 165624, 262131) disclose that micro-lens array is formed on one surface of a liquid crystal panel, for example. The micro-lens array shown here has micro-lenses corresponding to each picture element and condenses a light, which is conventionally blocked off by a black matrix into picture elements.
In addition to the above application, the micro-lenses are used in combination with the following means:
(1) condensing means of an optical pick-up, such as a laser disk, compact disk, magneto-optical disk;
(2) condensing means for combining an optical fiber with a light emitting element or a light receiving element;
(3) condensing means or image forming means for condensing an incident light to a photoelectric converting area so as to improve sensitivity of a solid image pickup element such as a CCD, or a one-dimensional image sensor which is used for a facsimile (for example, Japanese Unexamined Patent Publication 54-17620/1979 (Tokukaisho 54-17620), Japanese Unexamined Patent Publication 57-9180/1982 (Tokukaisho 57-9180);
(4) image forming means for forming an image to be printed on a photoreceptor in a liquid crystal printer or an LED printer (for example, Japanese Unexamined Patent Publication 63-44624/1988 (Tokukaisho 63-44624); and
(5) various optical elements or optical parts, etc. in an optical apparatus such as optical information processing filter.
As to such a method of producing a micro-lens, an ion exchange method (Appl. Optics, 21(6) p. 1052(1982), Electron Lett., 17 p. 452 (1981)), a swelling method (Suzuki and others "New Method of Producing a Plastic Micro-lens" 24th Institute of Micro-Optics), a heat sagging method (Zoran D. Popovic et al., Appl. Optics, 27 p. 1281 (1988)), machining method, etc. can be used.
In the ion exchange method, a plain type micro-lens array having a refractive index distribution type micro-lens can be obtained, and in the other methods, micro-lens array having a convex micro-lens can be obtained. In the case of especially the convex micro-lens array, as disclosed in Japanese Unexamined Patent Publication 5-134103/1993 (Tokukaihei 5-134103), a metal mold is produced by using the convex micro-lens array as a master, mass production of the micro-lens array is possible by using a method of molding a photosensitive resin by means of the metal mold (Photo Polymer method=2P method). An effective aperture ratio is improved by sticking the micro-lens array obtained by the above method to a liquid crystal panel, thereby obtaining a bright display picture.
In a liquid crystal panel which is used for projection TV which displays a high-precision picture at a picture element pitch of dozens .mu.m, an area of a picture element aperture is smaller compared to a liquid crystal panel in which the picture elements pitch is several hundred .mu.m. Since the effective aperture ratio is determined based upon a relationship between a size of the light spot of the micro-lens and the area of picture element aperture, the size of the light spot of the micro-lens, which is used for the liquid crystal panel having the picture element pitch of dozens .mu.m, should be smaller than that of the liquid crystal panel having the picture element pitch of several hundred .mu.m. This is because if the size of the light spot of the micro-lens is bigger than the area of the picture element aperture, a light which does not enter the picture element aperture cannot contribute to display, thereby lowering improvement of the effective aperture ratio by the micro-lens.
A relationship D=2.multidot.f.multidot.tan.theta. holds, wherein D is a diameter of the light spot, .theta. is the extent of the incident light angle (half width), f is the focal length of a micro-lens. In order to decrease the area of the light spot of a micro-lens, namely, improve an effect of condensing, it is considered that the extent of the incident light angle .theta. or the focal length of a micro-lens is shortened according to the above formula.
In order to decrease the extent of the incident light angle .theta., a light emitting area of a light source to be used is decrease and a distance from the light source to a panel is increased. However, it is hard to decrease the extent of the incident light angle .theta. to several degrees in order to secure a long service life and brightness required for display at a current technical level of the light source. Therefore, it is necessary to shorten the focal length of a micro-lens and position its focus in the vicinity of a picture element aperture of a liquid crystal panel (hereinafter, referred to as a short focus technique).
A panel in which a pitch of picture elements is 50 .mu.m and one side of a picture element aperture is about 30 .mu.m is produced by a current method of producing a liquid crystal panel. If an extent of an illumination light .theta. is 5.degree., in order to restrict the diameter of the light spot D to not more than 30 .mu.m, the focal length f should be not more than 170 .mu.m according to the above formula: D=2.multidot.f.multidot.tan.nu.. Meanwhile, since an amount of condensing of a light by a micro-lens is proportional to an area of the micro-lens, the amount of condensing of a light by the micro-lens becomes maximum in a state that the micro-lenses spreads all over at a same pitch as that of the picture elements, namely, when the diameter of the micro-lens is equal to the pitch of the picture elements. A numerical aperture (N.A.) of the micro-lens at this time becomes 0.147 according to N.A.=P/2.multidot.f. Therefore, in a high-precision liquid crystal panel in which a picture element pitch is dozens .mu.m, it is desirable that a value of numerical aperture for decreasing the light spot of a micro-lens is at least not less than 0.1.
Incidentally, in the above-mentioned micro-lens, cover glass with a thickness of 250 .mu.m which is applicable to a focal length in the air 170 .mu.m (a value obtained by multiplying a refractive index of glass and the focal length in the air) are put between the micro-lenses, and the focus should be positioned in the picture element aperture of the liquid crystal panel. In order to accomplish the above arrangement, a liquid crystal panel is produced such that the cover glass substrate with a thickness of 250 .mu.m is used as one of the substrates, and the micro-lens is stuck to the liquid crystal panel. However, this method makes it difficult to handle the cover glass substrate, so it is not unsuitable for mass production.
Japanese Unexamined Patent Publication 3-248125/1991 (Tokukaihei 3-248125) discloses a short focus technique of a micro-lens instead of the above method. In this method, a cover glass or film with a same thickness as a focal length is adhered to the surface of a micro-lens, and the micro-lens is produced in one substrate of a liquid crystal display element.
In addition, Japanese Unexamined Patent Publication 3-233417/1991 (Tokukaihei 3-233417) disclosed a method that a lens section is formed using a photosensitive resin by the 2P method and cover glass having a same thermal expansion coefficient as that of the micro-lens substrate is adhered to the lens section by adhesive having a different refractive index from that of the photosensitive resin so that mass productivity and adhesion are improved.
However, these prior arts causes the following problems because thin cover glass of 200 .mu.m to 300 .mu.m is used from the first.
First, when a cover glass is produced, since it is thin and very fragile, it is difficult to mass-produce the cover glass with a wide area at an excellent yield.
In addition, if cover glass is produced, a careful packing method is required in order to prevent damage during transportation.
Next, at the stage that a cover glass is stuck to a micro-lens, careful handling is required so as to prevent breakage of a cover glass. Moreover, in order to stick a cover glass to a micro-lens so as to have an uniform thickness, the cover glass should be pressed while the whole substrate is being uniformly loaded, but if even a slight flexing occurs, the cover glass breaks in its flexed place.
In addition, generally, in order to improve mass productivity, a plurality of liquid crystal display elements are taken out from one big glass substrate. Therefore, a plurality of micro-lenses are arranged on one glass substrate correspondingly to the liquid crystal display elements. A substrate for taking out a plurality of liquid crystal display elements to which such a micro-lens substrate is stuck is produced, and the liquid crystal display elements are cut one by one. However, since this substrate has an arrangement that three glass substrates are stacked up, in the case where the liquid crystal display elements are cut one by one, only glass on the outside can be cut along a prescribed line by a conventional cutting method. Therefore, the cover glass on the inside is not cut and is left, so the liquid crystal display elements cannot be cut one by one. Furthermore, when cutting, excessive force is applied to the cover glass, thereby arising a problem that the cover glass is liable to break, etc.